Rotating-gas distribution design

ABSTRACT

A burner includes a generally cylindrical casing ( 1 ) and a burner head ( 2 ) mounted within the casing, the burner head including one or more vanes ( 9 ) rotatably mounted on the head and rotatable about the head to sweep through a region between the burner head ( 2 ) and the casing ( 1 ). The vanes ( 9 ) each include one or more gas outlets for introducing gas into an air flow through the region.

This application claims priority to International Patent Application No.PCT/GB2008/004160 filed on Dec. 17, 2008, which claims priority to GreatBritain Patent Application No. 0724633.3 filed on Dec. 18, 2007.

This invention relates to a gas burner and to a method of operating agas burner, and more particularly, but not exclusively, to an industrialgas burner which may, for example, generate a thermal output of theorder of millions of BTU/hr (equivalent to hundreds of thousands or moreprobably millions of Watts).

Such a burner of a known kind comprises a generally cylindrical casingand a burner head mounted coaxially within the casing. In use air isblown past the burner head and gas, which has been fed to the burnerhead, is passed into the air flow passing the burner head. In order toburn the greatest possible amount of gas in the air flow, it isdesirable to have the best possible mixing of the gas emitted from theburner head with the air flow past the burner head. Many proposals havebeen made in the past to promote, for example, through the use ofdiffusers, intimate mixing of the air flow and the gas. As is of coursevery well known intimate mixing is enhanced by turbulence and it istherefore common to generate a turbulent region to promote good mixingof the gas and the air.

It is an object of the invention to provide a gas burner, and a methodof operating a gas burner, in which good mixing is achieved withoutresorting to the creation of considerable turbulence.

According to the invention there is provided a burner including agenerally cylindrical casing and a burner head mounted within thecasing, the burner head including one or more vanes rotatably mounted onthe head and rotatable about the head to sweep through a region betweenthe burner head and the casing, the one or more vanes each including oneor more gas outlets for introducing gas into an air flow through theregion.

By providing one or more rotatable vanes including gas outlets on theburner head it becomes possible to introduce gas into the air flowacross substantially the whole of the air flow path and therefore a verygood mixing of the gas and the air can be achieved solely by the way inwhich the gas is introduced.

It is a particular feature of the invention not to create undueturbulence through the introduction of the gas and it is accordinglypreferable for the vanes to be shaped such that their effect on the airflow is able to be reduced as a result of their rotation. This can beachieved by shaping the vanes, arranging them at an angle of pitch tothe airflow and then having them rotate in the direction that theairflow would cause them to rotate. Whilst it is within the broadestscope of the invention to have the vanes rotated solely by the airflowing past them, it is preferred that drive means, for example, anelectric motor, are provided for drivingly rotating the vanes.Preferably the vanes are rotated at a speed related to their pitch andthe speed of the airflow, such that in an axial direction the vanesprovide little or no resistance to the axial airflow. Of course, if thevanes are of constant pitch and the speed of the airflow isapproximately constant across the whole cross-section of the airflowthen the best rotational speed for the vanes will be higher in a morecentral region of the airflow than in an outer region. It would ifdesired be possible and within the scope of the invention to allow forthis variation by designing the vanes with a pitch that varied fromtheir inner ends to their outer ends but it is at present believed thatthe extra benefit achieved by introducing such a variation in pitchwould be small and it is therefore preferred not to do that.

Commonly it is desirable to operate a burner at a variety of firingrates and to control the gas and air flows through the burner accordingto the firing rates. Typically, an air blower is provided for creating aflow of air through the region swept by the one or more vanes. As thespeed of the airflow through the burner is varied, so the speed at whichthe vanes are drivingly rotated is desirably varied. Thus the burnerpreferably further includes a speed control for controlling the speed ofthe drive means for drivingly rotating the vanes. The burner alsopreferably includes a regulator for controlling the speed of the airflow through the burner. Such a regulator may control the speed of animpeller generating the airflow and/or it may control a valve memberpartially obstructing the air flow path. The gas flow rate may becontrolled by adjusting the pressure of gas at an inlet to the burnerhead. The burner preferably further includes a control system forcontrolling the speed of the drive means according to the speed of theairflow. Such a control system may be provided as a separate controlarrangement or may form an integral part of a control system for thewhole burner. The control system may make use of any of a variety ofinputs to control the speed of the drive means according to the speed ofthe airflow. For example a device for measuring the speed of the airflowmay be provided or, if the speed of the airflow is controlled independence upon the flow rate of gas, a signal indicating the flow rateof the gas may provide the input to the control system, or the speed ofthe drive means may be adjusted according to the pressure difference inthe airflow between a location upstream of the vanes and a locationdownstream of the vanes. That last approach is adopted in an embodimentof the invention, described below where the speed of the drive means isadjusted until there is no pressure difference between a locationimmediately upstream of the vanes and a location immediately downstreamof the vanes.

The one or more vanes may take any of a wide variety of forms notnecessarily having any aerodynamic shaping. Preferably a multiplicity ofvanes are provided equiangularly spaced apart around the burner head.For example, if nine vanes were provided, they would be spaced apart atintervals of 40 degrees. It may be preferable to provide more than ninevanes. For example there may be between 15 and 30 vanes and in anembodiment of the invention described below there are twenty vanesspaced at intervals of 18 degrees. The vanes preferably sweep throughsubstantially the whole of the cross-section through which the airflows.

Each of the one or more vanes preferably includes a multiplicity of airoutlets. For example, each vane may comprise a hollow member formed withmore than 10, and preferably more than 20, outlets. In an embodiment ofthe invention described below, there are 38 outlets on each vane.

The angle of pitch of the vanes may be chosen according to anyparticular application and what proves most desirable. Preferably theangle of pitch is in the range of 35 to 75 degrees. For example, thepitch angle could be about 45 degrees but it may also be reduced orincreased from that value; for example, it could be increased to about67 degrees. Of course, as already mentioned above, the angle of pitchmay vary along the length of each vane. For a given burner and a givenset of operating conditions the speed at which the vanes are preferablyrotated is of course reduced as the pitch of the vanes is increased.

Preferably each of the one or more vanes is formed from a tubularmember, which preferably extends substantially radially from the burnerhead. Preferably the outer end of the tubular member is closed and theinner end, mounted in the burner head, is open allowing gas to pass fromthe burner head into the tubular member. To facilitate attachment to theburner head, the inner end of the tubular member is preferably ofcircular cross-section, but to facilitate the formation of a vane therest of the tubular member is preferably of an oval cross-section. Theoval cross-section may readily be formed by deforming the tubular memberfrom an original circular cross-section. The deformation may be suchthat the longest dimension from one side to the other of the ovalcross-section is of the order of at least twice the shortest dimension.

The outlets are preferably formed at least mainly in the downstreamsides of the vanes. Preferably more outlets are provided in the regionof the outer ends of each vane than towards the inner end, to generatean even distribution of gas into the air flow. Of course, it is alsopossible to vary the sizes of the outlets, with bigger outlets in theregion of the outer end and smaller outlets in the region of the innerend.

The present invention also provides a method of operating a burner asdefined above, in which air flows through the region between the burnerhead and the casing and the one or more vanes rotate about the head,sweeping through the region between the burner head and the casing, andgas flows out through the one or more outlets in the one or more vanes.

By way of example, a burner embodying the invention will now bedescribed with reference to the accompanying drawings, of which:

FIG. 1 is a sectional side view of the burner which includes a burnerhead mounted therein;

FIG. 2 is a front view of the burner;

FIG. 3 is a perspective view of the burner head;

FIG. 4 is a side view of a gas outlet vane forming part of the burnerhead; and

FIG. 5 is an end view of the gas outlet vane.

Referring first to FIGS. 1 to 3, there is shown a burner including acasing 1 in a cylindrical part of which a burner head 2 is mounted. Theburner head 2 has a central portion coaxial with the cylindrical part ofthe casing 1 and is supported in the casing 1 at a rear end by a gasmanifold 3 and at a front end by three rods 4 that extend radiallyoutwardly from the burner head 2 to the casing 1.

In operation of the burner, air is driven through the casing 1 from theright to the left, as seen in FIG. 1, by an impeller (not shown). Gas tobe burnt is passed into the gas manifold 3, and passes along the burnerhead from right to left before being introduced into the airflow that ispassing through the casing 1 and around the burner head 2 at thedownstream (left as seen in FIG. 1) end of the burner head.

The features of the burner head described above are known per se. Theburner head 2 embodying the invention also includes certain specialfeatures: a shaft 5 is rotatably mounted along the central axis of theburner head 2 and carries a drive pulley 6 at its upstream end and avane assembly 7 at its downstream end. The vane assembly 7 includes anannular chamber 8 from which vanes formed by tubes 9 extend radiallyoutwardly terminating just inside and clear of the casing 1.

FIGS. 4 and 5 show one of the tubes 9 of the vane assembly 7. The tubehas an inner portion 10 of circular cross-section and an outer portion11 of oval cross-section. The outer portion 11 can conveniently beformed by deformation of the tube from a circular cross-section. Theouter portion 11 is closed by a cap (not shown) at its distal end and isprovided with a series of holes 12 defining gas outlets extending alongthe length of the tube, with more holes 12 being provided toward theouter end of the tube than toward the inner end.

In the example of the invention shown in FIGS. 4 and 5 the longestdimension of the oval cross-section of each tube 9 is inclined at anangle of about 45 degrees to the direction of air flow through thecasing 1, that direction being shown by arrows 15 in FIG. 5. As can alsobe seen from FIG. 5, the holes 12 are in the trailing region of theouter portion 11 of the tube 9 and the tube 9 is drivingly rotated inthe direction of arrow 16. In an alternative arrangement, in which thetube is set at a greater angle of pitch and is rotated more slowly, thelongest dimension of each tube is inclined at an angle of about 67.5degrees to the direction of arrow 16.

In operation the shaft 5 is drivingly rotated by an electric motor (notshown) driving the pulley 6 via a belt (not shown) and the tubes 9 arethereby driven around the burner head 2, sweeping through the annularspace between the chamber 8 and the casing 1. The direction of rotationof the tubes 9 and the direction in which the oval cross-sections of thetubes 9 are inclined to the airflow are those that would generate a flowof air in the direction in which it is actually flowing. Furthermore thetubes 9 are rotated at a speed related to the speed of the air flow sothat, viewed along a line parallel to the axis of rotation of the tubes9, the point of intersection of the line with the leading face of a tubemoves along the line at about the same velocity as the airflow.Consequently the resistance to the air flow is very much reduced and theturbulence also reduced. At the same time because the gas is introducedinto the air flow along the lengths of all the tubes which arethemselves rotating, the gas is well distributed throughout the airfloweven from the moment that it leaves the tubes.

As the firing rate of the burner is adjusted, so the gas and air flowrates are adjusted so that the speed of the air through the annularspace between the chamber 8 and the casing 1 is altered. It is thereforedesirable that the speed of rotation of the tubes 9 is also adjusted inproportion to the change in the speed of the air flow and for thatpurpose the speed of the electric motor driving the pulley 6 iscontrolled, for example via an inverter drive, according to the speed ofthe air flow. It is not unusual for a burner to operate over a ten toone range of fuel flow rates. As shown schematically in FIG. 1, pressuresensors 17 and 18 may be provided immediately upstream and downstream ofthe vanes. A control unit receiving signals from the pressure sensors 17and 18 is then able to adjust the speed of the drive from the pulley 6until the pressures measured at the sensors 17 and 18 are the same,indicating that the tubes 9 are neither braking nor accelerating the airflow.

As will be understood from the description above, there are many ways inwhich the design of the burner described above may be varied withoutdeparting from the invention. For example, the vanes may take a widevariety of shapes and sizes with openings in various locations. Thenumber and pitch of the vanes may be varied and the dimensions of theburner head and casing varied.

In one particular example of the invention, in which Natural Gas is thefuel, there are 20 tubes 9, each with 38 outlets formed by 1.2 mmdiameter holes. The pressure of the gas in the manifold 3 is maintainedat 3 psi (approximately 20684 Pa) leading to a firing rate at completecombustion of 21 million BTU/hr (approximately 6.3 million Watts). Withthat combustion rate an air flow of 224,000 cubic foot per hour(approximately 1,724,800×10⁻⁶ m³/s) is required resulting in an air flowspeed through the annular space between the chamber 8 and the casing 1of about 85 ft per second (approximately 26 m/s). With the tubes 9angled at 45 degrees to the airflow, the tubes are rotated at about 1660r.p.m. in order to match the air flow speed.

In another case, the example above is modified in that: each tube isprovided with 20 holes each of 1.5 mm diameter arranged in a single linebut with the spacing of the holes decreasing towards the outer end ofeach tube.

Whilst reference is made in the specification to mixing the fuel (gas)with “air” and in most applications it will be convenient simply to useambient air, it is of course within the scope of the invention to modifythe “air” prior to combustion. Also whilst the burner is necessarilycapable of burning gas as a fuel and the invention is concerned with theway in which the gas is introduced into an air flow it is also possiblefor the burner to have a facility for burning oil.

The invention claimed is:
 1. A method of operating a burner, in whichair flows through a region between a burner head and a casing and one ormore vanes rotate about the head, sweeping through the region betweenthe burner head and the easing, and gas flows out through one or moreoutlets in the one or more vanes, wherein the one or more vanes aredrivingly rotated by a drive means that comprises an electric motor, andthe speed of the drive means is controlled by a speed control, in whichthe one or more vanes are drivingly rotated at a speed that is varied asthe speed of the air flow between the burner head and the casing varies.